1. Field of the Invention
The invention relates to optics measuring and testing. More particularly, the invention relates to optical fiber cable measurement and inspection. The invention also relates to accessories for a fiber optic cable calibration standard. More particularly, the invention relates to spools for fiber optic cables.
2. Discussion of the Related Art
Optical time domain reflectometers (OTDR) are used to measure certain physical characteristics of fiber optic cables. The OTDR connects to only one end of a fiber optic cable. It sends a pulse of light into the fiber, which then travels down its length. Some light energy in the pulse is scattered and reflected back through the fiber optic cable and to the OTDR. Certain physical characteristics of the fiber optic cable reflect more energy back than others. The end of a fiber segment reflects back much more energy (called a Fresnel reflection) than the molecular structure of the fiber optic cable itself (Rayleigh scattering). The OTDR has an internal timer, which measures the time elapsed between the sending of the pulse, and any energy that is reflected and returned to the OTDR. If the speed of light in the fiber optic cable is known, the physical distance measured from the connection to the OTDR to the point where the light was reflected is calculated by:
  d  =                    c        f            ⁢      t        2  wherein: d is the physical distance to the point of reflection; cf is the speed of light in the medium of the fiber optic cable, and t is the elapsed time between the sending of the pulse by the OTDR and the reception of the reflected energy at the OTDR. The factor of 2 in the denominator accounts for the round-trip time t of the pulsed energy.
The speed of light in the fiber optic cable is dependent on the wavelength of the pulsed light. The relationship is given by:
      c    f    =      c          n      ⁡              (        λ        )            wherein: c is the speed of light in a vacuum (exactly 299,792,458 meters/second by definition of the meter), n(λ) is the index of refraction of the fiber optic cable at the wavelength, λ. The index of refraction is simply the ratio of the speed of light in a vacuum to the speed of light in the fiber optic medium, or:
      n    ⁡          (      λ      )        =      c          c      f      
Combining these equations gives the distance, d, in terms of the index of refraction, n, of the fiber, and the measured round-trip time, t, of the pulse of light in the fiber
  d  =                    c        ⁢                                  ⁢        t                    2        ⁢                  n          ⁡                      (            λ            )                                .  
A typical OTDR may include one or more pulsed light sources at one or more specific wavelengths. In order for the distance measurement of the OTDR to be meaningful, the index of refraction of the fiber to be measured must be entered into the OTDR, for the wavelength of the source used. With the index of refraction entered, the OTDR makes a time-based measurement, and calculates the distance using the above equation.
In the art, calibration of the OTDR is accomplished by using a sample spool of optical fiber of not well-known length, and not well-known index of refraction. The sample spool has been characterized for time-of-flight, or the time that a pulse of light of certain wavelength takes to travel through its length. The time-of-flight system is a time-based measurement standard. Once the time-of-flight is known for the sample spool, the OTDR is connected to it, and a distance measurement is performed. The result of the OTDR distance measurement will not correspond well with the actual physical length of the sample fiber. However, using the OTDR distance measurement and the index of refraction entered into the OTDR, a time-of-flight calculation can be made that shows the elapsed time measured by the OTDR. This equation is given by
  t  =                    2        ⁢                  n          ⁡                      (            λ            )                          ⁢        d            c        .  
Thus, the measured time-of-flight of the sample spool can be compared with the OTDR-measured time-of-flight, and the timing of the OTDR can be verified.
Commercially available fiber optic spools are 6-inch diameter, with 9-inch diameter retaining plates on either side and a width of 4.5-inch or more. The spools are constructed so that both ends of the fiber are accessible for use, which is important for time-of-flight measurements. However, when several spools of this size are used for a set of calibration standards, they take up a lot of space, which proves impractical for laboratory bench-top calibrations. Also, commercially available spools are made of assembled pieces fastened together. This design increases the chance that the fiber spooled onto it will get stuck in the cracks between the assembled pieces. Sticking in cracks adversely affects the reflective parameters of measurement spools. Commercially available spools are also relatively large in size.
From a practical standpoint, the verification of the OTDR timing can be difficult to interpret by the end-user. The end-user wants a distance measurement, not a time measurement. The calibration of the OTDR gives an optical fiber length in units of time, usually picoseconds, whereas the end-user would like to have length in meters or centimeters. Here-to-fore, there has been no standard available that allows for a direct length comparison between the OTDR distance measurement and a fiber optic distance standard.